This disclosure teaches techniques related to measurement of gas-concentration. Specifically, the teachings deal with measuring the variation in concentration gas residing in a target region.
Several conventional techniques have been developed for measurement of variations in gas concentration variation or gas flow-rate. These techniques include a dielectric relaxation method for measuring the dielectric constant of a substance, an absorption spectrum method for measuring the absorption distribution of electromagnetic waves, and an ultrasonic attenuation measuring method for measuring the amplitude decrement in ultrasounds after propagation. However, these techniques are generally insufficient at least in terms of time resolution.
A propagation-time-difference method is a conventional technique for measuring an acoustic propagation time between a transmitter and a receiver. This technique is simple and has a potential for achieving enhanced time resolution. In this technique, a standing wave is transmitted from a transmitter to a receiver. A propagation time is determined from a phase difference (phase shift) between the original standing wave in the transmitter and the received signal from the receiver. The variation in concentration or flow-rate of gas residing between the transmitter and the receiver is measured based on the propagation time.
For determining the above phase difference, signal processing on the side of the receiver is necessary to detect a time point when the amplitude of the received signal becomes greater than a given threshold. This is needed to determine a zero-cross point. However, the time point needs to be arranged to conform with the peak amplitude level of the received signal corresponding to that of the original standing wave. In addition, the amplitude of the received signal itself is undesirably fluctuated due to other physical factors, such as the temperature or humidity of the gas. This results in a large dispersion in the zero-cross points and the measured phase differences. This problem is referred to as xe2x80x9czero-cross problemxe2x80x9d herein.
In order to avoid the zero-cross problem, alternative methods have been conventionally proposed. These alternate methods includes a sing around method in which a received signal is returned to a transmitting end. The cycle of the returned signal is determined to determine the velocity of sonic wave. In a further alternate method, secondary/tertiary reflected waves between transmitting and receiving sections are detected to determine the velocity of sonic waves in a stable manner. In these methods, the variation in the concentration or flow rate of gas residing between the transmitting and receiving sections can be measured based on the determined sonic-wave velocity. These methods are free from the zero-cross problem because there is no need for determining the time point when the amplitude of a received signal becomes greater than a given threshold.
The sing around method or the sonic-wave velocity measuring method can provide a solution to the zero-cross problem. However, these methods take a substantial amount of time to complete their measurement over the entire period of the reflection time of sonic waves. This prevents the measurement of the variation in gas concentration in real time with a high time resolution.
The disclosed teachings are aimed at overcoming some of the above noted disadvantages.
To realize the advantages of the disclosed teachings there is provided a gas-concentration measuring apparatus for measuring the concentration variation of gas residing in a target region The gas-concentration measuring apparatus comprises an ultrasound-transmitter adapted to transmit an ultrasound in response to an ultrasound-generating signal having a rate of voltage change equal to or greater than a slew rate of an operational amplifier, and further adapted to output the ultrasound-generating signal. An ultrasound-receiver adapted to receive the ultrasound wave passed through the gas residing in the target region, and further adapted to convert the received ultrasound wave into an electrical signal serving as a received ultrasound signal is provided. A gas-concentration determiner adapted to input the ultrasound-generating signal and the received ultrasound signal into the operational amplifier to generate an amplified transmitting-side chopping wave and an amplified receiving-side chopping wave is further provided. The gas-concentration determiner is further adapted to compare the transmitting-side and receiving-side chopping waves independently with corresponding given threshold voltages to detect a first pair of time points when the respective chopping waves become equal to or greater than the corresponding given threshold voltages or to detect a second pair of time points when the respective chopping waves become equal to or less than the corresponding given threshold voltages. The gas-concentration determiner is further adapted to determine the concentration variation in the gas based on the pair of time points.
Another aspect of the disclosed teachings is a gas-concentration measuring method for measuring the concentration variation in gas residing in a target region. The gas-concentration measuring method comprises transmitting an ultrasound wave in response to an ultrasound-generating signal having a voltage change equal to or greater than a slew rate of an operational amplifier. The ultrasound wave passed through the gas residing in the target region is converted into an electrical signal serving as a received ultrasound signal. The ultrasound-generating signal and the received ultrasound signal are amplified to generate a transmitting-side chopping wave and an amplified receiving-side chopping wave. The transmitting-side and receiving-side chopping waves are compared independently with corresponding given threshold voltages to detect a first pair of time points when the respective chopping waves become equal to or greater than the corresponding given threshold voltages or a second pair of time points when the respective chopping waves become equal to or less than the corresponding given threshold voltages. The concentration variation in the gas is determined in accordance with the first and second pairs of time points.